JPA/Hibernate: @GeneratedValue doesnt work

I have an entity object with a technical id as primary key "BaseEntity" (its actual a superclass entity which provides the technical key for each sub-entity. Im using a Generic DAO pattern.

My Testcode looks like:

So, Person extends the BaseEntity above.

The persist() method throws the following exception:

Exception in thread "main" javax.persistence.PersistenceException: org.hibernate.exception.ConstraintViolationException: could not insert: [birthday.entities.Person]
at org.hibernate.ejb.AbstractEntityManagerImpl.throwPersistenceException(AbstractEntityManagerImpl.java:614)
at org.hibernate.ejb.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:226)
at birthday.dao.jpa.JpaDAO.persist(JpaDAO.java:40)
at birthday.dao.jpa.JpaDAO.persist(JpaDAO.java:1)
at birthday.test.TestDAO.main(TestDAO.java:38)